Put the chicken in a bowl. Pare strips of zest from the lemon using a vegetable peeler, then juice the lemon too. Add the peel and half the juice to the chicken, along with the oregano (if using), garlic, cinnamon and oil. Mix well, cover and chill for an hour. The lemon juice will start to ‘cook’ the chicken, so don’t leave for longer.
Heat the barbecue. If you are using coals, wait until they turn white. If you are indoors, heat a griddle pan. Thread the chicken strips onto a couple of metal skewers to stop them falling through the grate (you don’t need to do this for the griddle), then grill for a couple of mins each side. The strips will cook through quickly so don’t leave them too long. Season if you like.
Warm the flatbreads on the edge of the barbecue (or on the griddle) for a minute, then transfer them to plates and spread each with ½ tbsp yogurt. Divide the chicken strips between them, then dot on the remaining yogurt and sprinkle over the pepper and lettuce. Place another flatbread on top and then cut into 4 pieces.